1. Field of the Invention
The present invention relates to a planar light source device and a liquid crystal display device, and more specifically to a liquid crystal display device with enhanced viewing angle characteristics.
2. Description of Related Art
A liquid crystal display device has a liquid crystal panel comprising two substrates with a liquid crystal layer interposed therebetween. A planar light source device is mounted on the back of the liquid crystal panel. A twisted nematic (TN) mode is widely used in the liquid crystal panel, where liquid crystal molecules are aligned substantially parallel to the substrates in a quiescent state with a twist angle of given value and twist at the substrates according to an applied signal voltage.
A light source of the planar light source device is conventionally a linear cold-cathode tube. Recently, a light emitting diode (LED) also comes into use because of its longer life and good light emission. When using the LED as a light source, three LEDs of red, green, and blue are provided, each in a plurality of numbers. The angles of light from the three colors of the LEDs are substantially the same. The light emitting angle of light sources affects the viewing angle of the planar light source; a wider emission angle of light from the LED allows a wider viewing angle. Accordingly, the wavelength dependence of luminance at each angle of viewing direction in the conventional planar light source device using the LEDs is as shown in FIG. 11A. The luminance decreases as an angle of viewing direction becomes larger but remains substantially constant for any wavelength.
FIG. 11B shows the wavelength dependence of luminance at each angle of viewing direction in a conventional TN mode liquid crystal display device having the planar light source device using the LEDs. When viewing the liquid crystal display device from the same angle, the luminance of shorter wavelength light is lower relative to that of longer wavelength light, resulting in a reddish tone over all. Further, as the angle of viewing direction increases, the luminance differs more significantly depending on wavelength, causing color shift problem.
As described above, the conventional liquid crystal display device has a problem of color shift depending on angles of viewing direction.